1. Field of the Invention
The present invention relates to integrated circuits, and more particularly to such integrated circuits incorporating shielded inductor structures.
2. Description of the Related Art
Many modern integrated circuit devices, e.g., stable oscillators, require a high-Q (i.e., quality factor) inductor that is immune to external noise sources to achieve desired specifications. Crystal oscillators may be employed, but typically require an off-chip crystal mounted elsewhere on a printed-wiring-board. LC oscillators offer the potential advantage of being able to incorporate such an oscillator on-chip.
To achieve a suitable oscillator for certain applications (e.g, inclusion in a narrow bandwidth phase-locked loop (PLL)), a high-Q (i.e., quality factor) LC oscillator is typically required. For example, a Q>20 may be required for certain applications. It is difficult to achieve such a high-Q with conventional on-chip inductors using conductor and dielectric layer compositions and thicknesses which are typically encountered in traditional integrated circuit processes. In addition, such inductors are susceptible to electromagnetic interference from external sources of noise. For certain applications using LC oscillators, a low bandwidth PLL is desirable to ensure that jitter from a noisy source is not passed to the output. In contrast, high bandwidth PLLs tend to pass input jitter. However, the ability of a PLL to resist the pulling from external noise sources is directly proportional to the loop bandwidth. Inductors inside of the PLL, particularly inside an LC oscillator included in the PLL, are most prone to pulling. Accordingly, it is desirable to shield the inductor from external noise sources, particularly in low bandwidth applications to reduce the possible degradation in performance. Therefore, improvements to high-Q LC oscillators are desired to achieve stable oscillators, particularly for use as low-jitter clock sources.